1. Field of the Invention
The present invention relates to methods and apparatus for generating shear seismic waves in the ground during seismic prospecting.
2. Description of the Prior Art
French Pat. No. 2,276,599 discloses a method of generating shear seismic waves in the ground, which method includes driving a target member into the ground to a predetermined depth and generating a force in the target member by transmitting thereto impulses propagating in a plane generally parallel to the ground surface.
This method is practiced by an apparatus comprising a seismic energy source pivoted to a vehicle and including a target member to be driven into the ground to a predetermined depth and an impulse generator for transmitting the impulses to the target member in a plane generally parallel to the ground surface.
The target member in the prior art apparatus is a plate provided with tenons and adapted to be driven into the ground. The plate being seated down against the ground, a driven mass is being struck against the edge of the plate to thereby generate shear seismic waves in the ground.
Both the prior art method and the apparatus are limited in that they do not provide a satisfactory statistical effect in accumulation or recording of legitimate signals because of superposition of a constant and random noise. It is known that the statistical effect may be improved by increasing the frequency of source-point change but a desired increase in the source-point change is limited by the fact that the target member is first to be driven into the ground and only then the seismic wave generation is possible. Also, the rate of change of position of the prior art apparatus along the spread or the frequency of source-point change are dependent on the auxiliary operations such as conversion of the apparatus from a transport to an operating position and in the reverse order.
Furthermore, a larger amount of disturbing action on the ground at one source point results in comminution of the ground and impairment of its density, which decreases the level of the action and therefore the intensity of the shear wave generation is decreased.